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United States Patent |
5,171,333
|
Maurer
|
December 15, 1992
|
Methane purification by pressure swing adsorption
Abstract
A process is provided for the separation of ethane from methane-containing
feedstreams using pressure-swing adsorption wherein the adsorbent is a
faujasite type of zeolitic aluminosilicate containing at least 20
equivalent percent of calcium cations or zinc cations or mixtures thereof
and containing not more than 80 equivalent percent of alkali metal or
alkaline earth metal cations.
Inventors:
|
Maurer; Richard T. (Nanuet, NY)
|
Assignee:
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UOP (Des Plaines, IL)
|
Appl. No.:
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696383 |
Filed:
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May 6, 1991 |
Current U.S. Class: |
95/100; 95/103; 95/143; 95/902 |
Intern'l Class: |
B01D 053/04 |
Field of Search: |
55/25,26,31,33,58,62,68,74,75
|
References Cited
U.S. Patent Documents
2882244 | Apr., 1959 | Milton | 55/75.
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2988503 | Jun., 1961 | Milton et al. | 55/75.
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3078635 | Feb., 1963 | Milton | 55/75.
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3078639 | Feb., 1963 | Milton | 55/75.
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3078641 | Feb., 1963 | Milton | 55/75.
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3078642 | Feb., 1963 | Milton | 55/75.
|
3078644 | Feb., 1963 | Milton | 55/75.
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3130007 | Apr., 1964 | Breck | 23/113.
|
3176444 | Apr., 1965 | Kiyonaga | 55/26.
|
3200082 | Aug., 1965 | Breck et al. | 55/75.
|
3430418 | Mar., 1969 | Wagner | 55/25.
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3564816 | Feb., 1971 | Batta | 55/26.
|
3594983 | Jul., 1971 | Yearout | 55/33.
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3597169 | Aug., 1971 | Savage | 55/75.
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3636679 | Jan., 1972 | Batta | 55/62.
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3703068 | Nov., 1972 | Wagner | 55/21.
|
3738087 | Jun., 1973 | McCombs | 55/62.
|
3986849 | Oct., 1976 | Fuderer et al. | 55/25.
|
4240986 | Dec., 1980 | Priegnitz | 55/75.
|
4358297 | Nov., 1982 | Eberly, Jr. | 55/75.
|
4481018 | Nov., 1984 | Coe et al. | 55/75.
|
4503023 | Mar., 1985 | Breck et al. | 423/328.
|
4544378 | Oct., 1985 | Coe et al. | 55/75.
|
4557736 | Dec., 1985 | Sircar et al. | 55/75.
|
4599094 | Jul., 1986 | Werner et al. | 55/75.
|
4717398 | Jan., 1988 | Pearce | 55/75.
|
4775396 | Oct., 1988 | Rastelli et al. | 55/58.
|
4925460 | May., 1990 | Coe et al. | 55/75.
|
5013334 | May., 1991 | Maurer | 55/62.
|
Foreign Patent Documents |
55-030690 | Aug., 1980 | JP.
| |
Other References
Molecular Sieve Zeolites, Advances in Chemistry, Ser. 101, American
Chemical Society, Washington, D.C., 1971, p. 266.
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: McBride; Thomas K., Tolomei; John G., Silverman; Richard P.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. Ser. No. 462,514, filed
Jan. 1, 1990 and issued May 7, 1991 as U.S. Pat. No. 5,013,334.
Claims
What is claimed is:
1. A pressure swing adsorption process for the separation of ethane from a
feedstream comprising mixtures thereof with methane, said process
comprising:
a) passing the feedstream at an upper adsorption pressure through an
adsorption zone containing a zeolitic molecular sieve of the Y type having
a framework SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of from 2 to 100 and
containing at least 20 equivalent percent of calcium cations and
containing not more than 80 equivalent percent of alkali or alkaline earth
metal cations other than calcium, or mixtures thereof, wherein ethane and
methane are adsorbed and recovering an adsorption effluent stream enriched
in methane relative to the feedstream;
b) regenerating the adsorption zone by depressurizing the adsorption zone
to a desorption pressure, said desorption pressure being lower than said
adsorption pressure, and recovering a desorption effluent stream
comprising ethane and methane; and
c) repressurizing the adsorption zone to the adsorption pressure.
2. The process of claim 1 wherein said depressurizing is performed in a
direction countercurrent to said passing of said feedstream through said
adsorption zone.
3. The process of claim 2 which further includes at least one cocurrent
depressurization step performed prior to said regenerating step comprising
cocurrently depressurizing the adsorption zone to a pressure
intermediately between the adsorption pressure and the desorption pressure
and recovering a cocurrent depressurization effluent stream comprising
methane.
4. The process of claim 3 which includes three or more cocurrent
depressurization steps.
5. The process of claim 2 which further includes a countercurrent purge
step performed either prior to, simultaneously with, or subsequently to
said regenerating step, comprising passing a methane-containing purge gas
countercurrently through said adsorption zone and recovering a
countercurrent purge effluent stream comprising ethane.
6. The process of claim 2 wherein said process includes a first and second
adsorption zone and said repressurizing step comprises passing at least a
portion of one of the adsorption effluent stream or the cocurrent
depressurization effluent streams or both from said first adsorption zone
to said second adsorption zone.
7. The process of claim 1 wherein the framework SiO.sub.2 /Al.sub.2 O.sub.3
ratio of the zeolitic molecular sieve is from 2 to 20.
8. A process according to claim 7 wherein the zeolitic molecular sieve
contains at least 40 equivalent percent of calcium cations and less than
40 equivalent percent of alkali and alkaline earth metal cations other
than calcium or mixtures thereof.
9. The process of claim 1 wherein the feedstream additionally comprises at
least one component selected from the group consisting of C.sub.3 +
hydrocarbons, CO.sub.2, H.sub.2 O and mixtures thereof.
10. The process of claim 9 wherein the feedstream comprises from about 70
to 98 mol. % methane, from about 0.1 to 10 mol. % ethane, from about 0.1
to 10 mol. % C.sub.3 + hydrocarbons, and from about 0.1 to 20 mol. %
CO.sub.2.
11. The process of claim 9 wherein said components are adsorbed on the
zeolitic molecular sieve.
12. The process of claim 9 wherein the adsorption zone further comprises at
least two adsorbents wherein one adsorbent is selected from the group
consisting of silica gel, alumina, and activated carbon.
13. The process of claim 1 wherein the adsorption pressure is from about
100 to 500 psia.
14. The process of claim 1 wherein the desorption pressure is from about
14.7 to 50 psia.
15. A pressure swing adsorption process for the separation of ethane from a
feedstream comprising mixtures thereof with methane, said process
comprising:
a) passing the feedstream at an adsorption pressure of from about 100 to
500 psia to one of at least four adsorption zones containing a zeolitic
molecular sieve of the Y type having a framework SiO.sub.2 /Al.sub.2
O.sub.3 molar ratio of from 2 to 100 and containing at least 20 equivalent
percent of calcium cations and containing not more than 80 equivalent
percent of alkali or alkaline earth metal cations other than calcium, or
mixtures thereof; wherein ethane and methane are adsorbed and recovering
an adsorption effluent stream enriched in methane relative to the
feedstream;
b) cocurrently depressuring said one adsorption zone and passing the
effluent stream therefrom to another adsorption zone undergoing a
repressuring step as hereinafter described and equalizing the pressures
therebetween;
c) further cocurrently depressurizing said one adsorption zone and passing
the effluent stream therefrom as purge gas to another adsorption zone
undergoing a purging step as hereinafter described;
d) countercurrently depressurizing said one adsorption zone to a pressure
of from about 14.7 to 50 psia and recovering a desorption effluent stream
comprising ethane and methane;
e) countercurrently purging said one adsorption zone with a purge gas
comprising the cocurrent depressurization effluent stream of step (c) from
another adsorption zone and recovering a countercurrent purge effluent
stream comprising ethane;
f) repressurizing said one adsorption zone by passing the cocurrent
depressurization effluent stream of step (b) from another adsorption zone
to the adsorption zone to equalize the pressures therebetween; and
g) further repressurizing said one adsorption zone by passing a portion of
the adsorption effluent stream from another adsorption zone thereto.
16. The process of claim 15 wherein the feedstream comprises from about 75
to 95 mol. % methane, from about 0.5 to 10 mol. % ethane, from about 0.1
to 10 mol. % CO.sub.2 and about 0.1 to 5 mol. % C.sub.3 + hydrocarbons.
17. The process of claim 16 wherein the adsorption effluent stream
comprises less than about 2000 ppm ethane.
18. The process of claim 17 wherein the adsorption effluent stream
comprises less than about 300 ppm ethane.
Description
FIELD OF THE INVENTION
The present invention relates to processes for the separation of ethane
from feedstreams containing mixtures thereof with methane. More
particularly, the present invention relates to the use of particular
cation types of zeolitic molecular sieve having the faujasite type of
crystal structure as selective adsorbents in pressure swing adsorption
processes for methane purification.
BACKGROUND OF THE INVENTION
Pressure swing adsorption (PSA) provides an efficient and economical means
for separating a multi-component gas stream containing at least two gases
having different adsorption characteristics. The more strongly adsorbable
gas can be an impurity which is removed from the less strongly adsorbable
gas which is taken off as product; or, the more strongly adsorbable gas
can be the desired product, which is separated from the less strongly
adsorbable gas. For example, it may be desired to remove carbon monoxide
and light hydrocarbons from a hydrogen-containing feed stream to produce a
purified (99+%) hydrogen stream for a hydrocracking or other catalytic
process where these impurities could adversely affect the catalyst or the
reaction. On the other hand, it may be desired to recover more strongly
adsorbable gases, such as ethylene, from a feedstream to produce an
ethylene-rich product.
In pressure swing adsorption, a multi component gas is typically fed to at
least one of a plurality of adsorption zones at an elevated pressure
effective to adsorb at least one component, while at least one other
component passes through. At a defined time, the feedstream to the
adsorber is terminated and the adsorption zone is depressurized by one or
more cocurrent depressurization steps wherein pressure is reduced to a
defined level which permits the separated, less strongly adsorbed
component or components remaining in the adsorption zone to be drawn off
without significant concentration of the more strongly adsorbed
components. Then, the adsorption zone is depressurized by a countercurrent
depressurization step wherein the pressure on the adsorption zone is
further reduced by withdrawing desorbed gas countercurrently to the
direction of feedstream. Finally, the adsorption zone is purged and
repressurized. The final stage of repressurization is typically with
product gas and is often referred to as product repressurization.
In multi-zone systems there are typically additional steps, and those noted
above may be done in stages. U.S. Pat. Nos. 3,176,444 issued to Kiyonaga,
3,986,849 issued to Fuderer et al., and 3,430,418 and 3,703,068 both
issued to Wagner, among others, describe multi-zone, adiabatic pressure
swing adsorption systems employing both cocurrent and countercurrent
depressurization, and the disclosures of these patents are incorporated by
reference in their entireties.
Various classes of adsorbents are known to be suitable for use in PSA
systems, the selection of which is dependent upon the feedstream
components and other factors generally known to those skilled in the art.
In general, suitable adsorbents include molecular sieves, silica gel,
activated carbon and activated alumina. When PSA processes are used to
purify hydrogen-containing streams, the hydrogen is essentially not
adsorbed on the adsorbent. However, when purifying methane-containing
streams, methane is often adsorbed on the adsorbent along with the
impurity. The phenomenon is known in the PSA art as coadsorption.
The coadsorption of methane often causes a temperature rise in the
adsorption zone due to the exothermic heat of adsorption which can be
substantial, e.g., 40.degree. F. or more. The degree of temperature rise
depends, in part, upon the amount of methane present and the particular
adsorbent employed. As is well known in the art, such a temperature can be
undesirable since the equilibrium loading of many adsorbates, e.g.,
ethane, is reduced at higher temperatures. Hence it would be desirable to
reduce such temperature rises during adsorption. Similarly, during PSA
regeneration, the desorption of the coadsorbed methane along with the
impurity often causes a temperature decrease of about the same magnitude
as the previously mentioned temperature rise due to the endothermic heat
of desorption. Also, as known in the art, such a temperature decrease
during desorption can be undesirable since as noted above, the equilibrium
loading of many adsorbates, e.g., ethane, is reduced at higher
temperatures. Accordingly it would be desirable to reduce the temperature
decreases often observed during desorption. However, as can be discerned
from the above, the actual thermal behavior of the PSA system runs counter
to what is desired. Nevertheless, a variety of processes have been
proposed for purifying methane that utilize pressure swing adsorption.
U.S. Pat. No. 3,594,983 issued to Yearout et al., discloses a process for
the removal of ethane and other hydrocarbons in a natural gas feedstream,
i.e., methane, using a process that employes both pressure swing and
thermal swing adsorption. The patentees recognized that zeolitic molecular
sieves, e.g., 13X and 4A, were suitable for use in the process due to
their high affinity for the impurity components. The patentees' apparent
solution to the above-described thermal problem associated with methane
coadsorption was to incorporate the thermal swing step into the adsorption
process. That is, an adsorption zone that had been previously subjected to
pressure reduction, i.e., PSA, was thereafter heated to desorb remaining
impurities not removed by pressure swing.
Although not directly related to the separation of ethane from methane
containing streams, U.S. Pat. No. 4,775,396 issued to Rastelli et al.
discloses a PSA process for the bulk separation of CO.sub.2 from methane,
e.g., landfill gas. The patent discloses that for purification processes,
CO.sub.2 can be effectively removed from gas mixtures containing same
using the calcium ion-exchanged form of zeolite A, but because of the
strong affinity between the sorbent and adsorbate, thermal energy is
required for effective desorption of the CO.sub.2. However, for the bulk
removal of CO.sub.2 from methane, the patent discloses that PSA can be
effective when using faujasite type of zeolitic aluminosilicate containing
at least 20 equivalent percent of at least one cation species selected
from the group consisting of zinc, rare earth, hydrogen and ammonium and
containing not more than 80 equivalent percent of alkali metal or alkaline
earth metal cations.
Japanese Patent No. 1039163, issued Mar. 31, 1981 to Union Carbide Corp.,
discloses a process for the purification of methane by the removal of
ethane from a methane-containing feedstream that does not require the use
of a thermal swing regeneration step. The patent discloses a PSA process
that employs the use of silica gel as the adsorbent. The patent discloses
that the silica gel adsorbent provides (1) high differential loading for
all impurities to be removed from the product methane, (2) good enrichment
of impurities in the waste gas, and (3) ease of cleaning of the bed with
low pressure purge gas. It is further stated that high differential
loadings permit relatively small adsorption zones which are low in cost
and which reduce frequency of desorption, and hence reduce the product
loss associated therewith. Enrichment of impurities in the waste gas
reflects the degree of separation achievable in the process and is
important in order to reject the impurities with minimum loss of product
component. Ease of cleaning (or desorption) permits a high purity methane
product to be obtained with an economically small quantity of purge gas.
The above-identified characteristics stress the importance of achieving
both high purity and high recovery of methane.
As noted in the above-identified Japanese patent, it is desirable to
provide both high methane recoveries, i.e., minimum loss of product
component, and high purity, i.e., low ethane content. Generally, however,
there is an inverse relationship between purity and recovery and as such,
processes have been operated to provide a high purity product at low
recoveries or a low purity product at high recoveries. For example, it is
not uncommon to obtain less than 50% methane recovery when purities are
maintained at about 300 ppm ethane or less. Even at about 1000 ppm ethane
in the product methane, typical processes may only achieve about 55%
methane recovery. Accordingly, processes for the purification of methane
are sought which can provide a high purity product at higher recoveries
than heretofore possible.
SUMMARY OF THE INVENTION
By the present invention, a PSA process is provided for methane
purification that can yield a high purity product at higher recoveries
than heretofore possible. The process employs the use of a zeolitic
molecular sieve that can effectively control the thermal problems
associated with methane coadsorption yet provide suitable separation of
impurity adsorbates.
In a broad aspect of the present invention there is provided a pressure
swing adsorption process for the separation of ethane from a feedstream
containing mixtures thereof with methane. The process includes the steps
of a) passing the feedstream at an upper adsorption pressure to an
adsorption zone containing a zeolitic molecular sieve of the faujasite
type having a framework SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of from 2
to 100 and containing at least 20 equivalent percent of calcium or zinc
cations or mixtures thereof, and containing not more than 80 equivalent
percent of alkali or alkaline earth metal cations other than calcium, or
mixtures thereof, wherein ethane and methane are adsorbed and recovering
an adsorption effluent stream enriched in methane relative to the
feedstream; b) regenerating the adsorption zone by depressurizing the
adsorption zone to a desorption pressure that is lower than the adsorption
pressure, and recovering a desorption effluent stream containing ethane
and methane; and c) repressurizing the adsorption zone to the adsorption
pressure.
In another specific aspect of the present invention there is provided a
pressure swing adsorption process for the separation of ethane from a
feedstream containing mixtures thereof with methane. The process comprises
the steps of a) passing the feedstream at an adsorption pressure of from
about 100 to 500 psia to one of at least four adsorption zones containing
a zeolitic molecular sieve of the faujasite type having a framework
SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of from 2 to 100 and containing at
least 20 equivalent percent of calcium or zinc cations or mixtures
thereof, and containing not more than 80 equivalent percent of alkali or
alkaline earth metal cations other than calcium, or mixtures thereof;
wherein ethane and methane are adsorbed and recovering an adsorption
effuent stream enriched in methane relative to the feedstream, b)
cocurrently depressurizing one adsorption zone and passing the effluent
stream therefrom to another adsorption zone undergoing a repressurizing
step as hereinafter described and equalizing the pressures therebetween;
c) further cocurrently depressurizing the adsorption zone and passing the
effluent stream therefrom as a purge gas stream to another adsorption zone
undergoing a purging step as hereinafter described; d) countercurrently
depressurizing the adsorption zone to a pressure of from about 14.7 to 50
psia and recovering a desorption effluent stream comprising ethane and
methane; e) countercurrently purging the adsorption zone with the purge
gas stream comprising the cocurrent depressurization effluent stream of
step (c) from another adsorption zone and recovering a countercurrent
purge effluent stream comprising ethane; f) repressurizing the adsorption
zone by passing the cocurrent depressurization effluent stream of step (b)
from another adsorption zone to the adsorption zone to equalize the
pressures therebetween; g) further repressurizing the adsorption zone by
passing a portion of the adsorption effluent stream from another
adsorption zone thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the recovery and purity for the separation of ethane
from methane.
FIG. 2 is a schematic flowsheet of a four bed PSA system.
FIG. 3 is a chart illustrating the relative adsorption loading of ethane at
a pure component over a range of pressure for zinc X, calcium Y and silica
gel adsorbents.
FIG. 4 is a schematic design illustrating the composition profile of ethane
in a PSA bed during the adsorption and desorption steps.
FIG. 5 illustrates the recovery of methane in the PSA system as a function
of the amount of silica gel in the PSA bed.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention is useful for purifying
methane-containing streams. In general, typical feedstreams will contain
water up to saturation levels, less than 25 mol. % C.sub.2 -C.sub.5
hydrocarbons, less than 30 mol. % carbon dioxide and less than 2 mol. %
C.sub.6 and higher hydrocarbons. Natural gas is a common source of such
impurity-containing methane, and in natural gas the hydrocarbon impurities
are primarily the saturated type such as ethane and propane. Additionally,
natural gas can contain other components such as nitrogen, helium and
argon, although such other components are not appreciably adsorbed in the
process of the present invention. Preferably, the feedstream will contain
from about 70 to 98 mol. % methane, from about 0.1 to 10 mol. % ethane,
from about 0.1 to 10 mol. % C.sub.3 +hydrocarbons and from about 0.1 to 20
mol. % CO.sub.2. More preferably, the feedstream will comprise from about
75 to 95 mol. % methane, from about 0.5 to 10 mol. % ethane, from about
0.1 to 10 mol. % CO.sub.2, and from about 0.1 to 5 mol. % C.sub.3
+hydrocarbons.
The process is of the general PSA type wherein the methane feedstream is
introduced at the highest pressure to the inlet end of an adsorption zone
and the impurities are selectively adsorbed in each of at least two
sequentially operated adsorption zones. Impurity-depleted methane is
discharged from the adsorption zone so that impurity adsorption fronts are
formed in the zone at the feedstream inlet end and progressively move
toward the purified methane discharge end. Preferably, the
impurity-depleted methane product recovered as an adsorption effluent
contains less than about 2000 ppm of ethane and more preferably less than
300 ppm. The feedstream flow is terminated when the impurity adsorption
fronts are at a predetermined point between the zone inlet and discharge
ends. Impurity-depleted methane gas is then preferably released from the
adsorption zone discharge end thereby cocurrently depressurizing the
adsorption zone to lower pressure in one or more stages. The adsorption
zone is then countercurrently depressurized to the lowest desorption
pressure to remove the adsorbed impurities and adsorbed methane. The
depressurized zone is purged of the impurities by flowing one part of the
impurity-depleted methane gas from another adsorption zone
countercurrently therethrough from the discharge end to the inlet end. The
purged zone is at least partially repressurized by another adsorption zone
prior to the introduction of the feedstream to the inlet end. The term
"cocurrent" denotes that the direction of gas flow is cocurrent to the
direction of gas flow during the adsorption step. Similarly, the term
"countercurrent" denotes that the gas flow is countercurrent to the
direction of gas flow during the adsorption step.
The faujasite type of zeolite employed in the practice of the present
invention can be either of the type X or the type Y. Zeolite X and the
method for its preparation is described in detail in U.S. Pat. No.
2,882,244 issued Apr. 14, 1959 to R. M. Milton. The SiO.sub.2 /Al.sub.2
O.sub.3 molar ratio of zeolite X is from about 2 up to 3. In the as
synthesized form, zeolite Y has a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio
of from greater than 3 up to 6. The method for synthesizing zeolite Y is
disclosed in detail in U.S. Pat. No. 3,130,007 issued Apr. 21, 1964 to D.
W. Breck. The forms of zeolite Y which contain molar SiO.sub.2 /Al.sub.2
O.sub.3 values greater than 6 can be prepared by several dealumination
techniques well known in the art. For example, high temperature steaming
treatments which result in dealumination are reported by P. K. Maher et
al. in MOLECULAR SIEVE ZEOLITES, Advan. Chem. Ser. 101, American Chemical
Society, Washington, D.C., 1971, p. 266. A more recently reported
procedure, especially useful for increasing SiO.sub.2 /Al.sub.2 O.sub.3 of
zeolite Y, involves dealumination and the substitution of silicon into the
dealuminated lattice sites. This process is disclosed in U.S. Pat. No.
4,503,023 issued Mar. 5, 1985 to Skeels et al. As used herein, the term
"faujasite type of structure" means the framework structure, irrespective
of chemical composition, distrubution of the different T-atoms, cell
dimensions and symmetry, designated as "FAU" in the ATLAS OF ZEOLITE
STRUCTURE TYPES, W. M. Meier and D. H. Olsen, Published by the Structure
Commission of the International Zeolite Association (1978).
In order to be useful in the process of the present invention, the
faujasite zeolite must be treated in order to have a framework SiO.sub.2
/Al.sub.2 O.sub.3 molar ratio of from 2 to 100 and containing at least 20
equivalent percent of one or a mixture of two or more cation species
selected from the group consisting of zinc, rare earth, hydrogen,
ammononium and calcium and containing not more than 80 equivalent percent
of alkali or alkaline earth metal cations other than calcium or mixtures
thereof.
Preferably, the framework SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the zeolitic
molecular sieve is from 2 to 20 and the zeolitic molecular sieve contains
at least 20 equivalent percent of one or a mixture of two or more of zinc,
rare earth, hydrogen, ammonium and calcium cations.
It is further preferred that the zeolitic molecular sieve contains at least
40 equivalent percent of one or a mixture of two or more of zinc, rare
earth, hydrogen, ammonium and calcium cations and less than 40 equivalent
percent of alkali and alkaline earth metal cations other than calcium.
When Zeolite X is employed as the adsorbent, it is preferred that zinc
cations be used as an ion-exchange cation. When Zeolite Y is employed as
the adsorbent, it is preferred that calcium cations be used as an
ion-exchange cation. The various ion-exchange techniques and the like
which can be used to prepare the faujasite zeolites of the present
invention are well known to those skilled in the art and need not be
further described herein.
In addition to the faujasite zeolite, it may be desirable in certain
instances to employ another adsorbent in the adsorption zone in order to
remove some of the feedstream impurities prior to separating ethane from
methane with the faujasite zeolites. For instance, it may be desirable to
utilize an adsorbent bed of silica gel, activated carbon or alumina, or
combinations thereof, to adsorb the C.sub.3 + hydrocarbon fraction or
water. The use of such adsorbents and appropriate selection thereof can be
determined by those skilled in the art. Those skilled in the art will
further recognize that other adsorbents in addition to those specifically
enumerated above can be used in accordance with the present invention in
combination with the faujasite zeolite.
In accordance with the present invention, the adsorption pressure is
generally from about 50 to 1000 psia and preferably from about 100 to 500
psia. The desorption pressure is generally from about 1 to 100 psia and
preferably from about 14.7 to 50 psia. Suitable operating temperatures are
generally within the range of from about 50.degree.-150.degree. F. There
can be a variety of cocurrent depressurization steps to intermediate
pressures, cocurrent purging steps and countercurrent purging steps, all
of which are well known to those skilled in the art and described in the
previously cited patents relating to PSA processes. For example, three or
more of such cocurrent depressurization steps can be employed for pressure
equalization to further improve product recovery such as disclosed in the
above-identified U.S. Pat. No. 3,986,849. In addition, the purge step can
be performed either prior to, simultaneously with, or subsequently to the
regenerating or desorption step by passing a methane-containing stream
countercurrently through the adsorption zone and recovering a
countercurrent purge effluent stream comprising ethane. The
methane-containing stream can comprise a portion of the adsorption
effluent product stream. However, it is generally preferred that at least
a portion of one or more of the cocurrent depressurization effluent
streams be employed as the methane-containing stream, i.e., purge gas.
Moreover, the process of the present invention may be practiced with
essentially any adiabatic pressure swing process such as the multi zone
adsorption systems described in Fuderer et al., U.S. Pat. No. 3,986,849,
the four adsorption zone systems described in Wagner, U.S. Pat. No.
3,430,418 and Batta, U.S. Pat. No. 3,564,816, the three adsorption zone
systems described in Batta, U.S. Pat. No. 3,636,679, and the two
adsorption zone systems described in McCombs, U.S. Pat. No. 3,738,087, the
disclosures of which are hereby incorporated by reference.
The selection of the appropriate PSA process configuration and operating
conditions can be determined by those skilled in the art. For instance,
those skilled in the art will recognize that performing the adsorption
step at an excessively high pressures, i.e., which depend upon the
particular system and can be determined by experimentation, can produce
non-self-cleaning conditions and a large, abrupt increase in C.sub.2 +
content in the product.
As used herein, the term "self cleaning" is used in connection with
impurity enrichment. Self-cleaning is evidenced by a stable impurity front
which does not progressively encroach upon the product end of the
adsorption zone as numerous repetitive cycles proceed. If the adsorption
zone is not self-cleaning with respect to an impurity, then the impurity
will eventually appear in the effluent stream from the product end of the
adsorption zone and upon continued cycling, its concentration in the
product will gradually increase. In general, operating at lower pressures
during adsorption favors a product of higher purity (lower C.sub.2 +
content). However, as the adsorption pressure is reduced relative to the
purge pressure, differential loading on the adsorbent decreases and
adsorption zone size tends to increase (or cycle time must be shorter).
This increases the fraction of product lost in blowdown and required for
purging, and recovery of methane is reduced.
The process of the present invention will hereinafter be described with
reference to the drawings.
To further describe the role of the adsorbent in this process and its
selectivity for adsorbing ethane at various pressures, FIG. 1 illustrates
the relative ethane loading as a pure component for zinc X, calcium Y and
silica gel adsorbents in a PSA bed over a range of pressure on the x-axis
equivalent to the range of pressure between the desorption step at low
pressure and the adsorption step at high pressure. The differences between
the high pressure of the adsorption step and the low pressure of the
desorption step indicate the relative loading for ethane over the
particular adsorbent material. Table 1 shows that at the process
conditions of a typical PSA operation, zinc X and calcium Y both possess a
greater difference between ethane loadings over the pressure range of
operation than does silica gel. This difference is often called the delta
loading for a particular component and it indicates the relative amount of
working capacity in the PSA bed for the pressure swing operation. As shown
in the table, the delta loading for zinc X of 75 was about three times
that of the silica gel at 26 and the delta loading for calcium Y of 87 was
about 3.3 times that of the silica gel. In general, the greater the delta
loading, the greater the working capacity of the bed and the better
performance in the PSA system. Furthermore, these pure component data
indicate that both zinc X and calcium Y should have a similar working
capacity for ethane which would be greater than the silica gel alone over
these same conditions in a multicomponent system.
TABLE 1
______________________________________
PSA BED DELTA LOADING FOR PURE
ETHANE ADSORPTION
Ethane Loading
Ethane Loading
Delta
At End of At End of Ethane
Adsorbent Adsorption Step
Desorption Step
Loading
______________________________________
Type R-Silica Gel
29 3 26
Zinc X 82 7 75
Calcium Y 105 18 87
______________________________________
The pure component data in FIG. 1 represent data at a total pressure equal
to the partial pressure of ethane over the adsorbent. Thus, in the
operation of a PSA system processing a mixture of methane and ethane, the
partial pressure of ethane will be reduced significantly. Therefore, an
experiment was set up to evaluate the performance of these adsorbents
relative to the performance of the silica gel adsorbent.
FIG. 2 illustrates the composition profile of ethane as the partial
pressure of ethane which is the total pressure of the system multiplied by
the molar fraction of ethane in the bed at the end adsorption and at the
end of the desorption cycles in the PSA bed. During the adsorption step
wherein the adsorption of ethane is taking place at high pressure as the
feed (1) is introduced, the ethane profile (3) in the bed is at its
maximum. As methane product (2) is withdrawn, the ethane profile (3) moves
toward the product end of the bed. During the desorption step at low
pressure the ethane rich material is rejected from the bed as waste (4)
and the partial pressure of ethane (6) is reduced to its lowest level as
purge gas (5) is introduced.
FIG. 3 illustrates the recovery and purity relationship for the separation
of ethane from methane. The shaded area, designated curve A, was
reproduced from FIG. 5 of Japanese Patent No. 1039163 hereinbefore
described and represents the typical variation of product purity with
methane recovery observed by the patentees using silica gel adsorbent.
The data represented by the round data point shows typical performance
obtained by applicant when separating a feedstream containing about 6 mol.
% ethane, balance methane, using silica gel adsorbent in a PSA test cycle
having the steps of; adsorption at a pressure of about 130 psia, cocurrent
depressurization to a pressure of about 70 psia, cocurrent
depressurization to a pressure of about 33 psia, cocurrent
depressurization to a pressure of about 26 psia, countercurrent
depressurization to a pressure of about 16 psia, countercurrent purge with
product methane and repressurization to 130 psia. A single bed adsorption
zone was used for the testing and had a length of 5.9 ft. and an internal
volume of 0.03 ft.sup.3, i.e., about 850 cc. The adsorption zone was
insulated to approximate adiabatic operation. The adsorption zone was
loaded with silica gel adsorbent which is generally commercially available
and can be obtained from UOP, Des Plaines, Ill.
The data represented by the square data point illustrates the unexpectedly
enhanced results by replacing the 850 cc silica gel adsorption zone with a
zinc-exchanged zeolite X containing about 84 equivalent percent Zn.sup.+2
cations in accordance with the present invention. The zinc-exchanged
zeolite X can also be obtained from UOP, Des Plaines, Ill. The PSA test
cycle and testing unit were the same as described above. It can be seen
that for an ethane level of about 300 ppm in the product gas, the methane
recovery was about 29% higher, i.e., 66% versus 51%, than with the silica
gel.
Enhanced results were also observed when about 25 vol. %, i.e., about 212
cc, of the silica gel adsorbent from the effluent end of the adsorption
zone was replaced with zinc-exchanged Zeolite X. The PSA test cycle and
testing unit were the same as described above. At 300 ppm ethane in the
product gas, the recovery of methane was about 60% for the combined silica
gel-ZnX adsorption zone.
Recalling the trends in FIG. 1 for the calcium-exchanged zeolite Y and
their similarity with that of the zinc-exchanged zeolite X a series of
tests were performed in the simple bed adsorption zone for a partial
loading of the calcium-exchanged zeolite Y with silica gel to verify the
performance for the multicomponent separation system. Approximately 25
vol. % of the effluent end of the adsorption zone was replaced with
calcium-exchanged zeolite Y containing about 90 equivalent percent calcium
cations and the balance of the adsorption zone was silica gel. The
calcium-exchanged zeolite calcium Y can also be obtained from UOP, Des
Plaines, Ill. Although the density of the CaY material was 20% lower than
the ZnX material resulting in a lower amount of calcium cations on a
weight basis than the equivalent volume of the zinc X adsorbent, the
calcium-exchanged zeolite Y provide equally enhanced results. As in the
PSA cycle test with the zinc-exchanged zeolite X, for an ethane level of
300 ppm in the product gas, the recovery of methane was 57%, or about 12%
higher than the methane recovery for silica gel alone.
This experiment verified that the pure component adsorption trends relative
to the ZnX were valid for the CaY adsorbent even though the bed contained
a smaller amount of calcium-exchanged zeolite on a weight basis. FIG. 4
presents the performance of the PSA unit on a weight percentage of silica
gel in the bed for both the ZnX and the CaY zeolite adsorbents. The
performance data as measured by methane recovery in percent is plotted
against the weight percent of silica gel in the bed. Each adsorbent has
the base point for 100% silica gel in common at 51% methanol recovery. The
performance curve (1) described by the ZnX material reaches a 67% recovery
when the bed is 100% ZnX. On the basis of the performance of the CaY
material at slightly less than 25% of the bed and its pure component
relationship to the ZnX performance, the performance of the CaY material
at 100% of the bed will range between 57 and 63% or more likely between 61
to 63% methane recovery. Referring to FIG. 3, the data indicated by the
star is representative of the PSA operating with the bed operating with
100% CaY as developed from the above analysis.
Hence, it is possible to obtain a substantial improvement in performance
even when only a portion of the adsorption zone contains the adsorbent
material of the present invention.
The data in FIG. 3 represented by the triangular data point is presented
for comparison purposes and illustrates the expected results using a
faujasite that is not characterized in accordance with the present
invention. As determined by mathematical simulation of a PSA cycle as
described above, the expected performance using a sodium-exchanged zeolite
X, i.e., 13X, is about the same or worse than that of silica gel.
FIG. 5 is a schematic flowsheet of a four-adsorption zone PSA system. With
reference to FIG. 5, the four adsorption zones are loaded with a zinc
cation-exchanged zeolite X adsorbent obtained from UOP, Des Plaines, Ill.
The four adsorption zones are arranged in parallel between feedstream
manifold 10 and product manifold 11. Feed is introduced to manifold 10 and
passed through a heater 40 before entering feed header 41. Feed to, and
product from the adsorption zones are controlled respectively by automatic
valves 1A-D and 2A-D. A high pressure equalization step is accommodated by
connecting conduits 15 and 16 containing respectively automatic valves 4AB
and 4CD. Cocurrent depressurization for purge is accomplished through
manifolds 17 and 18 containing respectively automatic valves 5A-B and 5C-D
together with cross-over conduit 20 containing manual trim valve 22. A low
pressure equalization step is accomplished through cross-over conduit 21
which also connects manifolds 17 and 18 and contains automatic valve 19.
Flow from the adsorption zones through manifolds 13 and 14 to waste
manifold 12 is controlled by automatic valves 3A-D. Product for final
repressurization of the adsorption zones returns through conduit 27
containing regulating valves 23 and 28, then flows through
repressurization manifold 29 containing check valves 30 and 31, and
finally passes through one of manifolds 32 and 33 containing respectively
automatic valves 6A-B and 6C-D.
The full cycle will be described for Adsorption zone A and is typical for
all adsorption zones. Pressures and times are illustrative. Assume
adsorption zone A is pressurized and that all its associated valves are
initially closed. Valves 1A and 2A open and feedstream at 157 psia flows
from feedstream manifold 10 to the adsorption zone, and product flows from
the adsorption zone into manifold 11. Flow continues at steady feedstream
pressure for 5 minutes. Now valves 1A and 2A close and valve 4AB opens,
thereby establishing flow between adsorption zone A and adsorption zone B
which has been partially repressurized and is initially at 30 psia
pressure. The adsorption zones equalize in pressure at 97 psia in 0.75
minute. It is important to note that due to pressure drop through the
apparatus, the actual pressure in the two adsorption zones may, in fact,
not be exactly equal. Differences in the actual pressures are expected and
are within the scope of the present invention. Next, valve 4AB closes and
valves 5A, 5C, 3C and 26 open to establish flow between adsorption zones A
and C through manifold 20. Adsorption zone C has just completed
countercurrent blowdown and is now purged by gas from adsorption zone A
which is throttled by valve 22 to about 23 psia. Purge continues for 3.5
minutes when adsorption zone A pressure drops to 37 psia. PS-A, PS-B,
PS-C, and PS-D are pressure switches which sense the terminal pressure in
beds A-D, respectively. The terminal pressure is sensed by a pressure
switch PS-C which is actuated to close valve 3C and open valve 19. Flow
continues from adsorption zone A to adsorption zone C but now adsorption
zone C is dead-ended so that pressures equalize at 30 psia in 0.75 minute.
Valves 5A, 5C, 19 and 26 close and valve 3A opens, thus releasing residual
pressure in adsorption zone A countercurrently through waste manifold 12.
The final pressure, 23 psia, is set by a regulating valve (not shown)
downstream in waste conduit 12, which delivers the gas to a fuel header.
Valve 26 is a resistance valve rather than a shut-off valve and when
closed, imposes a flow restriction which prevents excess flow velocities
in adsorption zone A. Countercurrent depressurization is complete in 0.75
minute after which, valve 26, 5A and 5D open. This allows purge gas to
flow from adsorption zone D through manifold 20 to adsorption zone A at
about 23 psia, then through waste manifold 12. Purge continues for 3.5
minutes.
Adsorption zone A has completed its adsorption phase, its product recovery
phase and its desorption phase. It is now ready to begin a three-step
repressurization sequence. Valve 3A closes and flow from adsorption zone D
continues but with adsorption zone A dead-ended so that pressures equalize
at 30 psia in 0.75 minute. Valve 6A also opens to simultaneously permit
product from manifold 11 to return through valves 23 and 28 to adsorption
zone A. Now valves 5A and 5D close and valve 4AB opens. This establishes
communication with adsorption zone B which initially is at feedstream
pressure (157 psia), and the two adsorption zones equalize at 97 psia in
0.75 minute. Finally, valve 4AB closes and only product from manifold 11
continues to flow to adsorption zone A. The pressure in adsorption zone A
reaches substantially feedstream pressure (157 psia) in 4.25 minute.
This completes a full 20-minute cycle for adsorption zone A which is now
ready to receive the feedstream for the adsorption stroke by closing valve
6A and opening valves 1A and 2A. The cycle for adsorption zone A is
typical for all adsorption zones A-D, and the adsorption zones are placed
on adsorption sequentially in 1/4-cycle phase relationship such that the
feedstream and product flows are continuous.
The above-described process sets forth a four-adsorption zone five-step
cycle containing two pressure equalization steps. Alternatively, this
process may be practiced in a variety of other cycles as hereinbefore
described without departing from the scope of the claims that follow.
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